Method for improved resolution of patterning using binary masks with pupil filters

ABSTRACT

A photolithography lens system is disclosed. The system has several elements all perpendicularly aligned to an optical axis. The elements include a light source that generates an exposing light; a first lens that has a front focal plane and a pupil plane, and a binary mask between the light source and the first lens. The binary mask is placed at the front focal plane of the first lens. A pupil filter is placed at the pupil plane. Finally, a second lens is provided that has a front focal plane at substantially the same position as the pupil plane. The second lens also has a back focal plane where a semiconductor wafer is placed.

1. FIELD OF THE INVENTION

[0001] The present invention relates to photolithography ofsemiconductor devices, and more particularly, to the use of a pupilfilter in conjunction with a binary mask to improve resolution.

2. BACKGROUND INFORMATION

[0002] Photolithography is commonly used in a semiconductormanufacturing process to form patterns on a semiconductor wafer. In thephotolithography process, a photoresist layer is deposited over anunderlying layer that is to be etched. The photoresist layer is thenselectively exposed to radiation through a mask. The photoresist is thendeveloped and those portions of the photoresist that are exposed to theradiation are removed, in the case of “positive” photoresist.

[0003] The mask used to pattern the wafer is placed within aphotolithography exposure tool, commonly known as a “stepper”. In thestepper machine, the mask is placed between the radiation source and thewafer. The mask is typically formed from patterned chromium placed on aquartz substrate. The radiation passes through the quartz sections ofthe mask where there is no chromium substantially unattenuated. Incontrast, the radiation does not pass through the chromium portions ofthe mask. Because radiation incident on the mask either completelypasses through the quartz sections or is completely blocked by thechromium sections, this type of mask is referred to as a binary mask.After the radiation selectively passes through the mask, the pattern onthe mask is transferred onto the photoresist by projecting an image ofthe mask onto the photoresist through a series of lenses.

[0004] As features on the mask become closer and closer together,diffraction effects begin to take effect when the size of the featureson the mask are comparable to the wavelength of the light source.Diffraction blurs the image projected onto the photoresist, resulting inpoor resolution.

[0005] One prior art method of preventing diffraction patterns frominterfering with the desired patterning of the photoresist is to coverselected openings in the mask with a transparent layer that shifts oneof the sets of exposing rays out of phase, which will null theinterference pattern from diffraction. This approach is referred to as aphase shift mask (PSM). Nevertheless, use of the phase shift mask hasseveral disadvantages. First, the design of a phase shift mask is arelatively complicated procedure that requires significant resources.Secondly, because of the nature of a phase shift mask, it is difficultto check whether or not defects are present in the phase shift mask.

[0006] Another prior art approach is to use attenuated phase shift masks(AttPSM) to enhance resolution. The AttPSM has “leaky” chrome featuresthat are partially transmitting. Additionally, the light in the quartzregion is phase shifted by 180 degrees. The attenuated phase shift maskoperates by attenuating the zero order of light. However, onedisadvantage of attenuated phase shift masks is their cost ofmanufacture. Additionally, it has been found that attenuated phase shiftmasks can create an undesirable resist loss at the side lobes of thecontacts. The diffraction pattern of a square contact at the wafer,known as the Airy disk, consists of a main central intensity peak andsmaller secondary peaks that are offset from the main peak. When usingAttPSM, these secondary peaks are in phase with the background electricfield. The intensity resulting from the constructive interaction can besufficient to expose the resist, creating the undesired features knownas side lobes.

SUMMARY OF THE INVENTION

[0007] A photolithography lens system is disclosed. The system hasseveral elements all perpendicularly aligned to an optical axis. Theelements include a light source that generates an exposing light, afirst lens that has a front focal plane and a pupil plane, and a binarymask between the light source and the first lens. The binary mask isplaced at the front focal plane of the first lens. A pupil filter isplaced at the pupil plane. Finally, a second lens is provided that has afront focal plane at substantially the same position as the pupil plane.The second lens also has a back focal plane.

BRIEF DESCRIPTION OF DRAWINGS

[0008] The present invention will be described in conjunction with thefollowing drawings, wherein:

[0009]FIG. 1 is a schematic diagram of a prior art lens system forexposing a semiconductor wafer during photolithography.

[0010]FIG. 2 is a schematic diagram of a lens system for exposing asemiconductor wafer during photolithography formed in accordance withthe present invention.

[0011]FIG. 3 is an illustration of a pupil filter formed in accordancewith the present invention.

[0012]FIG. 4 is a graph of the transmissivity characteristics of thepupil filter of FIG. 3.

[0013]FIG. 5 is a schematic illustration of a three slit pattern.

[0014]FIG. 6 is a schematic illustration of a two-dimensional holepattern.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention uses a first focusing lens, a pupil filter,and a second focusing lens to produce an image of a binary mask patternwith sharper defined edges on a semiconductor wafer. Additionally, theterm “binary mask” refers to those masks that only have regions that aresubstantially opaque and regions that are substantially transmissive.The light emitting from a light source passes though the binary mask,the first lens, the pupil filter, and the second lens, and then projectsan image of the binary mask pattern onto the semiconductor wafer. Thefirst lens produces a Fourier-transformed image of the mask pattern. Thepupil filter selectively adjusts the amplitude of theFourier-transformed image to produce an “attenuated Fourier-transformed”image. The second lens produces an inverse-Fourier transformed image ofthe attenuated Fourier-transformed image, which is then projected ontothe wafer. As will be described below in more detail, theinverse-Fourier transform of the attenuated Fourier-transformed image isan accurate replica of the original mask pattern with sharply definededges.

[0016] When the openings of a mask that defines the mask pattern havedimensions comparable to the wavelength of the light source, diffractionwill occur when the light passes through the openings on the mask andonto the wafer. One example of such an opening is a contact hole, whichis square on the mask, but due to diffraction, the image of the openingformed on the wafer is blurred at the edges and prints as a roundfeature. The light intensity will be higher near the center of the slitimage, decreasing gradually at the edges. Thus, the boundaries of theimage of the opening at the wafer will not be clearly defined.

[0017] Fourier Analysis

[0018] Referring to FIG. 1, a light source 101, a first lens 103, and asecond lens 105 are aligned along the optical axis 107 of the lenses 103and 105. The focal lengths of the first lens 103 and the second lens 105are both equal to f. An object plane 109 is situated at the front focalplane of the first lens 103. The front direction refers to the directiontowards the light source 101. A pupil plane 111 is situated at the backfocal plane of the first lens 103. The pupil plane 111 is also situatedat the front focal plane of the second lens 105. An image plane 113 issituated at the back focal plane of the second lens 105. As seen in FIG.1, the distance between the object plane 109 and the center of the firstlens 103 is f, the distance between the center of the first lens 103 andthe pupil plane 111 is f, and the distance between the pupil plane 111and the center of the second lens 105 is f. Finally, the distancebetween the center of the second lens 105 and the image plane 113 isalso f.

[0019] For purpose of illustration, assume that the x-axis is thehorizontal axis (in the direction into the Figure), the y-axis is thevertical axis, and the z-axis is the optical axis 107. A two-dimensionalpattern u(x, y) is placed at the object plane 109. According to Fourieroptics theory, the image formed at the pupil plane 111 is thetwo-dimensional Fourier transform of u(x, y), which is represented byU(fx, fy). The intensity, U²(fx,fy), is referred to as the Fraunhoferdiffraction pattern. The symbols fx and fy represent the coordinates onthe pupil plane 111. The relationship between the u(x, y) and U(fx, fy)can be written as (Eq. 1):

U(fx, fy)=F[u(x, y)]

[0020] The notation F[ ] represents the Fourier transform operator.

[0021] When the image U(fx, fy) passes through the second lens 105 andis projected on the image plane 113, the image at the image plane 113will be the inverse-Fourier transform of the image formed at the pupilplane 111. If nothing is placed at the pupil plane 111 to alter theamplitude and phase of the image at pupil plane 111, then the imageprojected on the image plane 113 is nominally the original pattern u(x,y). This is because the inverse-Fourier transform of aFourier-transformed image is the same image itself. This can be writtenas:

F ⁻¹ [F[u(x, y)]]=u(x, y)

[0022] The notation F⁻¹[ ] represents the inverse-Fourier transformoperator. In reality, due to the finite size of lenses, not all of thediffraction orders (Fourier modes) in the pupil plane can be collected.Hence, the image does not exactly match the object.

[0023] Photolithography Using Pupil Filter in the Spatial FrequencyPlane

[0024] Turning to FIG. 2, a schematic illustration of an embodiment ofthe present invention is shown. A photolithography system 201 includes alight source 203, a binary mask 205, a first lens 207, a pupil filter211, a second lens 209, and a wafer 213 that are all aligned along theoptical axis 215. The mask 205, first lens 207, pupil filter 211, secondlens 209, and the wafer 213 are placed perpendicularly to the opticalaxis 215. The light source 203 is typically an ultraviolet (UV) or deepultraviolet (DUV) light source, although it may be any type of radiationsource normally used in photolithography. An example of the light source203 is a KrF laser emitting DUV radiation with a wavelength of 248 nm.All components of FIG. 3, except for existence and placement of thepupil filter 211, are of conventional design for many photolithographystepper machines.

[0025] The binary mask 205 is typically formed of deposited chromium onquartz in accordance with conventional techniques. The binary mask 205carries a mask pattern 330 that is to be imprinted onto the wafer. Thewafer 213 is typically coated with a photoresist layer, so that afterthe photolithography process, a replica of the mask pattern 330 isformed on the photoresist layer on the wafer 213. The binary mask 205,the first lens 207, the pupil filter 211, the second lens 209, and thewafer 213 are mounted on a support frame of the photolithographicmachine that is not shown in the FIG. 2.

[0026] The focal length of the first lens 207 and the second lens 209are equal to f. The binary mask 205 is situated between the light source203 and the first lens 207. The first lens 207 has two focal planes. Thefront focal plane 217 of the first lens 207 is defined to be the onethat is closer to the light source 203, and the back focal plane isdefined to be the one that is farther away from the light source 203.Likewise, the second lens 209 has two focal planes. The front focalplane of the second lens 209 is defined as the one that is closer to thelight source 203, and the back focal plane 219 is defined as the onethat is farther away from the light source 203. In this embodiment, theback focal plane of the first lens 207 coincides with the front focalplane of the second lens 209, and is called the pupil plane 221. This isbecause the image formed at the back focal plane of the first lens 207is the Fourier transform of the image at the front focal plane 217.

[0027] In operation, light from the light source 203 passes through thebinary mask 205, passes through the first lens 207, the pupil filter211, the second lens 209, and then projects an image upon the wafer 213.The first and second lenses 207 and 209 are conventional focusingoptical lenses commonly used in many of the photolithography machines.The center of the pupil filter 211 is situated at the pupil plane 221.The wafer 213 is situated at the back focal plane 219 of the second lens209.

[0028] Preferably, the pupil filter 211 is formed using conventionaltechniques. For example, the paper “Optimization of Pupil Filters forIncreased Depth of Focus”, by von Bunau et al., Jpn. J. Appl. Phys.,Vol. 32 (1993) pp. 5350-5355 discusses various methods of manufacturingpupil filters. Specifically, for circularly symmetric transmissionpattern, as discussed in the von Bunau paper, one method is to evaporatea metal film through a stencil mask onto a rotating substrate.

[0029] Being located at the pupil plane 221, the pupil filter 211 actsdirectly on the spectral components of the image of the binary mask 205to redistribute the relative intensities of the diffraction orders.Specifically, the pupil filter 211 acts to suppress the zero and firstorder of light emerging from said binary mask 205. The present inventionattempts to emulate the effect of an attenuated phase shift mask,without the cost and other disadvantages of the attenuated phase shiftmask.

[0030] Thus, the pupil filter 211 working in conjunction with the binarymask 205 should have the same effect as an “attenuated phase shift maskversion” of the binary mask 205. In other words, the pupil filter 211and the binary mask 205 should be equivalent to the binary mask 205converted using conventional techniques into an attenuated phase shiftmask. In mathematical terms:

[Binary Mask]×[Pupil Filter]=AttPSM

[0031] or

Pupil Filter=AttPSM/[Binary Mask]

[0032] From the above equation, the design of the pupil filter 211requires the analysis of the Fraunhofer diffraction pattern of thebinary mask and the AttPSM. The following expression gives the electricfield at the pupil plane 221 of a single slit of width “2a” for aconventional binary mask, mask transmission function F(x)=2a,

U(p)=C ₁ ∫F(x)e ^(−ikpx) dx=C ₁×2asinc(kpa),

[0033] where sinc(ξ)=sin(ξ)/ξ, k=2π/λ, and p=ξNA/f(NA=Numerical apertureof the lens, f=focal length).

[0034] For AttPSM, the analysis is extended for a repeated 3 slitpattern whose transmission amplitude and phase are given by A₁₋₃ andφ₁₋₃, respectively. The width of the center slit is 2a and the widths ofthe adjacent slits are (b−a) (see FIG. 5). Thus,${U(P)} = {C_{1}\begin{bmatrix}{{\left( {b - a} \right)A_{1}^{{\varphi}_{1}}^{\quad {{kp}{(\frac{a + b}{2})}}}\sin \quad {c\left( {{{kp}\left( {b - a} \right)}/2} \right)}} +} \\{{2{aA}_{2}^{{\varphi}_{2}}\sin \quad {c({kpa})}} +} \\{\left( {b - a} \right)A_{3}^{{\varphi}_{3}}^{{- }\quad {{kp}{(\frac{a + b}{2})}}}\sin \quad {c\left( {{{kp}\left( {b - a} \right)}/2} \right)}}\end{bmatrix}}$

[0035] For AttPSM, A₁=A₃, φ₁=φ₃=π(180°), A₂=1, φ₂=0. Hence, the aboveexpression can be simplified to give the electric field of thediffracted mask pattern at the pupil plane 221 as a function of maskparameters: pitch (2b), feature size (2a), background transmissionamplitude A₁, and the exposure wavelength ( ).

AttPSM U(P)=C ₁×[2asinc(kpa)−2(b−a)A ₁ sinc(kp(b−a)/2)cos(kp(a+b)/2)]

[0036] Using the above equations, the pupil filter 211 to be used withbinary mask 205 to give a diffraction pattern that closely approximatesthe attenuated phase shift mask version of the binary mask 205 can beobtained explicitly in terms of mask and stepper parameters as:

[Binary Mask]×[Pupil Filter]=AttPSM

[0037] or

Pupil Filter=AttPSM/[Binary Mask]

[0038] ∴PupilFilter=1−{[2(b−a)A₁sinc(kp(b−a)/2)cos(kp(a+b)/2)]/2asinc(kpa)}

[0039] For a two dimensional (holes instead of slits) representation ofthe AttPSM (as shown in FIG. 6), the electric field is given by:${U(P)} = {C_{1}\begin{Bmatrix}{{2c\quad \sin \quad {{c({kqc})}\left\lbrack {{2a\quad \sin \quad {c({kpa})}} - {2\left( {b - a} \right)A_{1}\sin \quad {c\left( {{{kp}\left( {b - a} \right)}/2} \right)}{\cos \left( {{{kp}\left( {a + b} \right)}/2} \right)}}} \right\rbrack}} -} \\{2b\quad \sin \quad {{c({kpb})}\left\lbrack {A_{1}2\left( {d - c} \right)\sin \quad {c\left( {{{kq}\left( {d - c} \right)}/2} \right)}{\cos \left( {{{kq}\left( {c + d} \right)}/2} \right)}} \right\rbrack}}\end{Bmatrix}}$

[0040] Hence, the equivalent pupil filter is${2a\quad \sin \quad {c({kpa})}2c\quad \sin \quad {c({kqc})} \times {PF}} = {{\begin{Bmatrix}{{2c\quad \sin \quad {{c({kqc})}\left\lbrack {{2a\quad \sin \quad {c({kpa})}} - {2\left( {b - a} \right)A_{1}\sin \quad {c\left( {{{kp}\left( {b - a} \right)}/2} \right)}{\cos \left( {{{kp}\left( {a + b} \right)}/2} \right)}}} \right\rbrack}} -} \\{2b\quad \sin \quad {{c({kpb})}\left\lbrack {A_{1}2\left( {d - c} \right)\sin \quad {c\left( {{{kq}\left( {d - c} \right)}/2} \right)}{\cos \left( {{{kq}\left( {c + d} \right)}/2} \right)}} \right\rbrack}}\end{Bmatrix}\therefore{PF}} = {1 - \left\lbrack {A_{1}\frac{\left( {b - a} \right)}{a} \times \frac{\sin \quad {c\left( {{{kp}\left( {b - a} \right)}/2} \right)}{\cos \left( {{{kp}\left( {a + b} \right)}/2} \right)}}{\sin \quad {c({kpa})}}} \right\rbrack - {\quad\left\lbrack {\frac{A_{1}{b\left( {d - c} \right)}}{a\quad c} \times \frac{\sin \quad {c({kpb})}\sin \quad {c\left( {{{kq}\left( {d - c} \right)}/2} \right)}{\cos \left( {{{kq}\left( {c + d} \right)}/2} \right)}}{\sin \quad {c({kpa})}\sin \quad {c({kqc})}}} \right\rbrack}}}$

[0041] Typically for contacts, c=a, and d=b. Therefore,${\therefore{PF}} = {1 - \left\lbrack {A_{1}\frac{\left( {b - a} \right)}{a} \times \frac{\sin \quad {c\left( {{{kp}\left( {b - a} \right)}/2} \right)}{\cos \left( {{{kp}\left( {a + b} \right)}/2} \right)}}{\sin \quad {c({kpa})}}} \right\rbrack - {\quad\left\lbrack {\frac{A_{1}{b\left( {b - a} \right)}}{{a\quad}^{2}} \times \frac{\sin \quad {c({kpb})}\sin \quad {c\left( {{{kq}\left( {b - a} \right)}/2} \right)}{\cos \left( {{{kq}\left( {a + b} \right)}/2} \right)}}{\sin \quad {c({kpa})}\sin \quad {c({kqa})}}} \right\rbrack}}$

[0042] This equation defines how the electric field of a conventionalbinary mask is modulated in the pupil plane when using AttPSM. The samefield modulation can be achieved using a conventional binary mask andmodulating the transmission and phase directly in the pupil planethrough a pupil filter. The equation for PF defines the transmission andphase of the filter at all points (p,q) in the pupil plane to achievethe modulation imparted by the AttPSM.

[0043] Since the diffraction patterns of the mask pattern for the pupilfilter and AttPSM are identical by design, the resolution enhancementsto patterning are also identical. By substituting values for a (halfwidth of feature), b (half period), and A₁ (transmission amplitude ofthe background) in the equation above for the pupil filter, a pupilfilter equivalent to an AttPSM can be obtained. A variety of pupilfilters can be designed for various combinations of a, b, and Al. Thisanalytical technique gives a method of parameterizing the family ofpupil filters to find an optimum for the desired configuration.

[0044] Using the above formula, it has been found that the PF for anisolated feature (b>>a) has a phase and transmittance variation. It isdesirable to have a pupil filter without any phase change since phasedefects add to lens aberrations and the filters are also difficult tomanufacture. The pupil filter for a tightly nested feature where b˜2a isa pure transmittance filter (no phase change) which results inresolution enhancement through the suppression of the zero order lightIn some cases the absolute value of PF can be >1. This is not physicallypossible. PF is then scaled so that the maximum transmittance is 1. Thiswill result in a difference in the peak image intensity for the binarymask+PF vs. the equivalent AttPSM which the PF was meant to mimic.

[0045]FIG. 3 illustrates an exemplary pupil filter 211 formed inaccordance with the present invention. FIG. 4 shows a graph illustratingthe transmissivity of the pupil filter 211 relative to radial positionoff of the optical axis 215. As can be seen, the central area of thepupil filter 211 is more opaque to the irradiating light than theperiphery. In FIG. 4, the radial position is measured in units of/NA,where NA is the numerical aperture of the first lens 207. The amplitudescale of FIG. 4 is scaled to have a value of 1.0 for completetransmissivity and 0.0 for complete opaqueness. The graph of FIG. 4 istaken directly from calculated data where A₁=0.4242 (18% transmissionintensity), b=110 nm, a=55 nm. The image produced at the back focalplane of the first lens 207 is the Fourier transform of the image at thefront focal plane 217. Assuming that the thickness of the pupil filter211 is small compared with the focal length f, the image projected ontothe front end of the pupil filter 211 is the Fourier-transformed imageof the mask pattern of the binary mask 205. The pupil filter 211selectively changes the amplitude of the Fourier-transformed image, andproduces an “attenuated Fourier-transformed” image of the mask pattern.The image formed on the back focal plane 219 of the second lens 209 isthe inverse-Fourier transform of the image at the front focal plane ofthe second lens 209. Thus, the image projected onto the wafer 213 is theinverse-Fourier transform of the attenuated Fourier-transformed image ofthe mask pattern.

[0046] Assume the mask 205 has a two-dimensional mask pattern 330 thatis described as u(x, y). The image u(x, y) is situated at the frontfocal plane 217 of the first lens 207. The Fourier-transformed image atthe front end of the pupil filter 211 is U₀(f_(x), f_(y)), where fx, fyare the coordinates on the spatial frequency plane. The image formedafter passing through the pupil filter 211 is U₁(fx, fy).

[0047] The pupil filter 211 is near the front focal plane of the secondlens 209 (under the assumption that the thickness of the pupil filter211 is small compared with the focal length f). The image projected onthe back focal plane 219 is the inverse Fourier transform of the imageat the front focal plane of the second lens 209. Therefore, thecombination of the first lens 207, pupil filter 211, and second lens 209has the effect of transferring the image of the mask pattern 330 ontothe wafer 213 with the edges more sharply defined. The blurring due todiffraction is reduced accordingly.

[0048] The above description of illustrated embodiments of the inventionis not intended to be exhaustive or to limit the invention to theprecise forms disclosed. While specific embodiments of, and examplesfor, the invention are described herein for illustrative purposes,various equivalent modifications are possible within the scope of theinvention, as those skilled in the relevant art will recognize.

[0049] These modifications can be made to the invention in light of theabove detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope of the invention is to be determined entirely by the followingclaims, which are to be construed in accordance with establisheddoctrines of claim interpretation.

What is claimed is:
 1. An apparatus comprising: a light source thatgenerates an exposing light; a first lens placed perpendicular to anoptical axis, said first lens having a front focal plane and a pupilplane; a binary mask placed perpendicular to said optical axis andbetween said light source and said first lens, said binary mask placedat said front focal plane of said first lens; a pupil filter placedperpendicular to said optical axis and at said pupil plane; and a secondlens placed perpendicular to said optical axis, said second lens havinga front focal plane at substantially the same position as said pupilplane, said second lens also having a back focal plane.
 2. The apparatusof claim 1 further including a semiconductor wafer placed at the backfocal plane of said second lens.
 3. The apparatus of claim 1 wherein thedistance between said pupil filter and said first lens is substantiallyequal to the focal length of said first lens, and the distance betweensaid pupil filter and said second lens is substantially equal to thefocal length of said second lens.
 4. The apparatus of claim 1 whereinsaid pupil filter is formed in accordance with: Pupil Filter=1−{[2(b−a)A₁ sinc(kp(b−a)/2)cos(kp(a+b)/2)]/2asinc(kpa)}where A₁ is thetransmission amplitude of said binary mask, is the wavelength of saidexposing light, k=2/, a is one-half of a feature size of said binarymask, p is the pitch of said binary mask, and b=p/2.
 5. The apparatus ofclaim 1 wherein said pupil filter acts to suppress the zero order oflight passing through said binary mask.
 6. An apparatus comprising: alight source that generates an exposing light; a first lens placedperpendicular to an optical axis, said first lens having a front focalplane and a pupil plane; a binary mask placed perpendicular to saidoptical axis and between said light source and said first lens, saidbinary mask placed at said front focal plane of said first lens; a pupilfilter placed perpendicular to said optical axis and at said pupilplane, said pupil filter formed such that the combination of said pupilfilter, said first lens, and said binary mask modify said exposing lightto emulate the effect of an attenuated phase shift mask version of saidbinary filter and said first lens acting on said exposing light; and asecond lens placed perpendicular to said optical axis, said second lenshaving a front focal plane at substantially the same position as saidpupil plane, said second lens also having a back focal plane.
 7. Theapparatus of claim 6 wherein said pupil filter is formed in accordancewith: Pupil Filter=1−{[2(b−a)A ₁sinc(kp(b−a)/2)cos(kp(a+b)/2)]/2asinc(kpa)}where A₁ is the transmissionamplitude of said binary mask, is the wavelength of said exposing light,k=2/, a is one-half of a feature size of said binary mask, p is thepitch of said binary mask, and b=p/2.
 8. The apparatus of claim 7wherein said pupil filter acts to suppress the zero order of lightpassing through said binary mask.
 9. The apparatus of claim 6 furtherincluding a semiconductor wafer placed at the back focal plane of saidsecond lens.
 10. The apparatus of claim 6 wherein the distance betweensaid pupil filter and said first lens is substantially equal to thefocal length of said first lens, and the distance between said pupilfilter and said second lens is substantially equal to the focal lengthof said second lens.
 11. A method comprising: forming a pupil filter inaccordance with: Pupil Filter=1−{[2(b−a)A ₁sinc(kp(b−a)/2)cos(kp(a+b)/2)]/2asinc(kpa)} where A₁ is the transmissionamplitude of said binary mask, is the wavelength of said exposing light,k=2/, a is one-half of a feature size of said binary mask, p is thepitch of said binary mask, and b=p/2.
 12. The method of claim 11 furthercomprising placing said pupil filter at the pupil plane of a first lens.13. The method of claim 12 further comprising placing a binary mask at afront focal plane of said first lens.
 14. The method of claim 13 furthercomprising providing incident light onto said binary mask such that saidincident light is selectively passed through said binary mask, saidfirst lens, and said pupil filter.
 15. The method of claim 14 furthercomprising placing a second lens having a front focal plane and an imageplane such that said front focal plane substantially coincides with saidpupil plane.
 16. The method of claim 15 further comprising placing asemiconductor wafer at said image plane.
 17. A method for exposing asemiconductor wafer for a photolithography process comprising: placingsaid semiconductor wafer at an image plane of a second lens, said secondlens having a front focal plane; placing a pupil filter at said frontfocal plane of said second lens; placing a first lens having a frontfocal plane and a pupil plane, said pupil plane substantially coincidentwith said pupil filter; placing a binary mask at said front focal planeof said first lens; and illuminating said binary mask with an incidentlight such that said incident light selectively passes through saidbinary mask, said first lens, said pupil filter, and said second lensonto said semiconductor wafer.
 18. The method of claim 17 wherein saidpupil filter is designed to suppress the zero order of said incidentlight emerging from said binary mask.
 19. The method of claim 17 whereinsaid pupil filter is designed in accordance with: PupilFilter=1−{[2(b−a)A ₁ sinc(kp(b−a)/2)cos(kp(a+b)/2)]/2asinc(kpa)}where A₁is the transmission amplitude of said binary mask, is the wavelength ofsaid exposing light, k=2/, a is one-half of a feature size of saidbinary mask, p is the pitch of said binary mask, and b=p/2.
 20. Themethod of claim 17 wherein said pupil filter is formed such that thecombination of said pupil filter, said first lens, and said binary maskmodify said incident light to emulate the effect of an attenuated phaseshift mask version of said binary filter and said first lens acting onsaid incident light.